    $(document).ready(function($){
    // browser window scroll (in pixels) after which the "back to top" link is shown
    var offset = 300,
        //duration of the top scrolling animation (in ms)
        scroll_top_duration = 200,
        //grab the "back to top" link
        $back_to_top = $('.cd-top');

    //hide or show the "back to top" link
    $(window).scroll(function(){
        ( $(this).scrollTop() > offset ) ? $back_to_top.addClass('cd-is-visible') : $back_to_top.removeClass('cd-is-visible cd-fade-out');
    });
    //smooth scroll to top
    $back_to_top.on('click', function(event){
        event.preventDefault();
        $('body,html').animate({
            scrollTop: 0 ,
            }, scroll_top_duration
        );
    });

});